1. Field of the Invention
The invention relates to calibration equipment for testing the accuracy and consistency of rotary power assembly tools for threaded fasteners, and in particular for impulse tools.
2. Background Art
Our earlier filed PCT patent application published as WO98/10260 discloses a simulated test joint for use with power assembly tools such as power screwdrivers, torque wrenches, pneumatic nut runners and hydraulic impulse tools. It discloses a variable rate test joint comprising a housing, a shaft mounted within the housing, means for coupling a tool to be tested to the shaft, a brake shoe assembly for contacting the shaft to apply a frictional torque to the shaft, and a computer for controlling the magnitude of the braking torque applied to the shaft as a function of time. By modelling the increase of braking torque on the shaft as a time-dependent function rather than an angle-dependent function the test joint of WO98/10260 proves itself suitable for the calibration of impulse tools as well as of constant torque tools such as torque wrenches.
The universally accepted sequence of phases in the calibration of any constant torque power tool is as follows:
Phase 1: Run the power tool so as to operate the test joint without any external load until the power tool reaches its normal free-running speed. This phase simulates the xe2x80x9crun-downxe2x80x9d of a threaded fastener, in which a nut, for example, is advanced along a screw thread towards engagement with an abutment surface of an article to be fastened. There is no resistance to rotary movement other than the inevitable but small friction losses which would occur during the xe2x80x9crun-downxe2x80x9d of the threaded fastener.
Phase 2: Apply a friction torque up to about 5% to 10% of the rated maximum torque for the tool. This simulates the fastener being driven to its xe2x80x9csnugxe2x80x9d condition in which all free play has been absorbed.
Phase 3: Increase the torque applied by the friction brake, following a prescribed torque/angle relationship which is a straight line characteristic according to ISO 5393. The gradient of the torque/angle relationship defines whether the simulated joint is hard or soft in accordance with ISO 5393 which defines four grades of torque rate graded A to D. This phase of progressive torque increase commences at the end of the above Phase 2.
Termination: Measurements stop when the tool and simulated joint stop rotating. This condition is unambiguously identifiable when the tool being tested has an automatic shut-off mechanism, but for manually controlled tools becomes a matter of judgement. If an operator judges the moment of xe2x80x9cstoppingxe2x80x9d too early, the results of a test will be very variable. If an operator judges the moment of xe2x80x9cstoppingxe2x80x9d too late, the simulated test joint will have locked solid and the tool will be reacting against a solid and immovable object, which is likely to record an unrealistically high torque.
The above criteria, although developed for constant torque tools such as torque wrenches or power screwdrivers, have also been applied without modification to impulse tools. For a constant torque tool the tool could if desired be held without the application of torque for a period between Phase 2 and Phase 3, or the torque ramp of Phase 3 could begin immediately after Phase 2 is complete, with the fastener driven to its xe2x80x9csnugxe2x80x9d condition. For an impulse tool of course the tool flywheel is already rotating at maximum speed when Phase 2 finishes, so immediate progression to Phase 3 is necessary. There are other more significant differences between the behaviour of constant torque tools and impulse tools, and this invention is based on the fact that impulse tools behave differently, so that modified criteria are necessary before the simulated test joint can provide a truly accurate calibration of the impulse tool.
In particular, this invention is based on the realization that for pulse tools the assessment of completion of Phase 2 and of Termination is subject to considerable uncertainty. When a pulse tool is run in free six (Phase 1), it will reach very high speeds. Five thousand rpm is not unknown. When running at such high speeds, some pulse tools will deliver one very large pulse or a series of very large pulses on encountering an initial and possibly very small torsional resistance. That initial torsional resistance may be due to initial and possibly uncharacteristic engagement between the brake shoes and the shaft or may even be due to a side loading on the impulse tool as it is held on the simulated joint, and unrelated to the braking torque applied to the rotary shaft. During this time, the speed of the pulse tool will reduce and it will tend not to deliver any more pulses until it encounters greater torsional resistance. In these circumstances, there is a period of very significant instability during the transition from free-running to normal pulsing of the pulse tool. Only when the proper pulsing mode is securely established will torque pulses be developed by the tool at regular intervals and of increasing magnitude, to react against an increasing braking torque applied to the shaft. No known means for identifying the completion of Phase 2 in the above test sequence is able to compensate for the period of instability during the transition from Phase 1 to Phase 2. It is an object of the invention to overcome the above problems and to provide calibration apparatus for calibrating pulse tools which overcomes the above problems. It is a further object of the invention to provide such calibration apparatus that is capable of calibrating both pulse tools and constant torque assembly tools with equal accuracy.
The Invention
The invention provides an apparatus for calibrating rotary power assembly impulse tools, comprising a variable rate simulated test joint in which a rotary shaft is braked by brake shoes in direct contact with the shaft under the control of a computer; means for gradually increasing the braking torque applied to the shaft by the brake shoes while the shaft is driven at free-running speed by the tool being calibrated; means for monitoring the pulsed output of the impulse tool; means for recognizing from the monitored output a regular stream of output pulses from the impulse tool and for identifying the first pulse of that regular stream; and means for increasing the magnitude of the braking torque as a linear function of time with a predefined gradient representative of the torque rate of the joint being simulated, commencing with the identified first pulse.
The recognition of the regular stream of output pulses from the impulse tool can be achieved by recognizing the establishment of n successive pulses at equal or substantially equal time intervals t, where n is an integer greater than 2, preferably 3 or 4, and t is preferably approximately the natural pulsing frequency of the tool.
The n pulses may be monitored as torque pulses sensed using a torque transducer on or associated with the shaft of the simulated test joint, in which case the pulses are counted as only those pulses in excess of a given torque threshold. However some pulse tools produce their torque output pulses not as single pulses but as pulse pairs one or both of which may be over the given torque threshold but which together result in a single continuous angular movement of the shaft. Such pulse pairs may be perceived and counted as either single pulses or double pulses depending on the frequency response of the monitoring means, but according to this invention are preferably counted as single pulses. Therefore it is clear that careful attention to selection of the most appropriate frequency response is necessary when monitoring torque pulses according to the invention.
Alternatively the n pulses may be monitored as angle pulses sensed using an angle encoder associated with the shaft of the simulated test joint. Typically the angle encoder is one that recognizes angular shaft movements of for example 0.5xc2x0, each 0.5xc2x0 of shaft movement producing a leading or trailing edge in a square waveform output. Therefore a complete pulse cycle would represent 1xc2x0 of shaft movement between repeating positive-going transitions in the output waveform. If for example a single torque pulse resulted in a 3xc2x0 shaft movement, then that shaft movement would be monitored as a single angular event as represented by a closely grouped set of three complete pulses in the output waveform, each pulse having one leading edge and one trailing edge.
It may be desirable to count the number of output pulses of the impulse tool during the above Phase 3 of the calibration sequence, since this information can be of use in assessing and grading the performance of the assembly tool. Such counting is readily achievable according to the invention since it is a simple count of the monitored output pulses from the impulse tool. Preferably however when such counting is a feature of the apparatus of the invention the pulses being monitored and counted are pulses of xe2x80x98single angular eventsxe2x80x99 as discussed above.
The same apparatus may be used in accordance with teachings of WO98/10260 for the calibration of rotary power assembly constant torque tools.
The apparatus of the invention advantageously further includes means for identifying the termination of the test sequence accurately and using essentially the same analytical techniques as those used to determine the transition from Phase 2 to Phase 3. The pulses monitored may be torque or angle pulses. The means for monitoring the pulsed output of the impulse tool continues its analysis during the whole of the test cycle, and preferably includes means for identifying the last pulse of that regular stream. After that last identified regular pulse, the pulse output of the tool becomes erratic, indicating that the simulated test joint has locked solid so that the impulse tool is reacting against a solid and immovable object. Monitoring is continued until the regularity of the pulsed output is lost, but all measurements made after the identified last regular pulse are ignored. This provides reliability of the calibration even when the tool does not have automatic shutoff and when the test continues after the shaft has locked fully. If on the other hand the user terminates such a test prematurely, before the shaft has locked fully, then the continued monitoring of the pulsed output of the impulse tool will indicate to the computer that the termination condition has not yet been reached, in which case the apparatus should preferably automatically indicate that the calibration is null and void.